This invention relates to antennas to receive signals from Global Positioning System (GPS) satellites and, more specifically to antenna systems arranged for reception for differential GPS applications.
Antenna systems providing a circular polarization characteristic in all directions horizontally and upward from the horizon, with a sharp cut-off characteristic below the horizon are described in U.S. Pat. No. 5,534,882, issued to A. R. Lopez on Jul. 9, 1996 (which may be referred to as “the '882 patent”). Antennas having such characteristics are particularly suited to reception of signals from GPS satellites.
As described in that patent, application of the GPS for aircraft precision approach and landing guidance is subject to various local and other errors limiting accuracy. Implementation of Differential GPS (DGPS) can provide local corrections to improve accuracy at one or more airports in a localized geographical area. A DGPS ground installation provides corrections for errors, such as ionospheric, tropospheric and satellite clock and ephemeris errors, effective for local use. The ground station may use one or more GPS reception antennas having suitable antenna pattern characteristics. Of particular significance is the desirability of antennas having the characteristic of a unitary phase center of accurately determined position, to permit precision determinations of phase of received signals and avoid introduction of phase discrepancies. Antenna systems having the desired characteristics are described and illustrated in the '882 patent, which is hereby incorporated herein by reference.
For such applications, antennas utilizing a stack of individually-excited progressive-phase-omnidirectional elements are described in U.S. Pat. No. 6,201,510, issued to A. R. Lopez, R. J. Kumpfbeck and E. M. Newman on Mar. 13, 2001 (“the '510 patent”). Elements as described therein include self-contained four-dipole elements which are employed in stacked configuration to provide omnidirectional coverage from the zenith (90 degrees elevation) to the horizon (0 degrees) or from a high elevation angle to the horizon, with a sharp pattern cut off below the horizon. The '510 patent is hereby incorporated herein by reference.
Objects of the present invention are to provide new and improved antennas and methods, including antennas and methods usable for DGPS applications and which may provide one or more of the following characteristics and advantages:
vertically-steerable null;
null steerable for low elevation interference suppression;
adaptively controlled null steering capability;
omnidirectional azimuth coverage with elevation coverage up from the horizon;
wide frequency band operability;
progressive-phase-omnidirectional azimuth pattern; and
operable with circularly polarized signals.